The invention relates to an injection molding form and to a process for producing closed or open hollow bodies of plastic according to the preamble of the main claim as well as the preamble of the accompanying process claim.
An injection molding form for producing closed or open hollow bodies of rubber or rubber-like synthetic resin is disclosed in DE-OS 21 13 628. In this form the core is supported by a retractable movable stop. The injection of the rubber or rubber-like material takes place at the support point. The mouth of the injection passage into the mold cavity is disposed in the area of the movable stop, for example in the form of an annular groove encompassing the free stop end within the bore in the corresponding molding. A disadvantage of this injection molding form consists in the fact that the local thickening formed by filling the freed opening in the core must then be removed again. This necessitates a subsequent mechanical processing step.
Furthermore, DE-GM 86 22 028 discloses an apparatus for holding cores in a casting mold. This apparatus provided that spacers be disposed which fix the core in the casting mold. These spacers are rings which surround the core or ring segments which partially surround it, which are in contact with the inner wall of the casting mold cavity and are composed of a thermoplastic synthetic resin whose melting range is the same as that of the encompassing material. A disadvantage of the arrangement of rings is that they must first be pushed onto the core, and especially in the case of irregularly shaped cores this is not possible without additional work. Moreover, these rings remain in the plastic molding which is subsequently formed and constitute a weak point since the bond between the ring and the injection molded material is not assured.
Also disclosed in DE-OS 39 20 100 is a process and an apparatus for the injection molding of a box-like container in whose wall cavities are provided running perpendicular to its standing surface. To form the cavities, elongated spars are provided in the apparatus which are supported by longitudinally movable pins to stabilize their position. To prevent holes from remaining in the completed container the pins are drawn away from the spars in accordance with the level of the injection pressure prevailing in their vicinity which changes as the level of the plastic present in their area increases, until their ends are flush with the surface of the part of the apparatus out of which they protrude. A disadvantage of this process is that, after the pins are withdrawn, the position of the core can change. The pins thus do not assure a uniform plastic structure.